1. Field of the Invention
The present invention relates to a liquid crystal display and a driving method thereof, and more particularly, to a liquid crystal display (LCD) and a driving method thereof which enable stable operation at a low temperature.
2. Description of the Related Art
As personal computers and TVs have recently become lighter and thinner, display devices need to become lighter and thinner as well. Accordingly, flat panel display devices such as an LCD have been developed and substituted for the cathode ray tube (CRT).
In the LCD, a liquid crystal layer with dielectric anisotropy is injected between two substrates of a panel, and the light transmittivity of the panel is controlled by applying and controlling an electric field. Desired images are obtained in such a manner.
The LCD is the most common representative display among flat panel displays, and in particular the thin film transistor liquid crystal display (TFT-LCD) employing the TFT as a switching element is most widely used.
Each pixel of the TFT-LCD can be modeled as a liquid crystal capacitor because electrodes (pixel electrodes and a common electrode) are arranged facing each other with liquid crystal inserted therebetween. Each pixel of the TFT-LCD can be indicated as an equivalent circuit such as shown in FIG. 1.
As shown in FIG. 1, each pixel of an LCD includes a TFT 10 having a source electrode coupled to a data line Dm and a gate electrode coupled to a scan line Sn, a liquid crystal capacitor Cl coupled between a drain electrode of the TFT 10 and a common electrode Vcom, and a storage capacitor Cst coupled to the drain electrode of the TFT 10.
The TFT 10, responding to a scan signal from the scan line Sn, supplies the data voltage Vd from the data line Dm to each pixel electrode (not illustrated) through the TFT 10. Then, an electric field corresponding to a difference between the pixel voltage Vp supplied to the pixel electrode and the common voltage Vcom supplied to the common electrode is supplied to the liquid crystal (shown as the liquid crystal capacitor Cl equivalently in FIG. 1). The storage capacitor Cst maintains the pixel voltage Vp supplied to the liquid crystal capacitor Cl until the next data voltage is supplied so that a desired amount of light can be transmitted through the liquid crystal during one period.
In general, LCDs can be classified according to the method of indicating color images into two groups. The methods include a color filter method and a field sequential driving method.
An LCD using the color filter method has a color filter layer which has the three primary colors of red R, green G, and blue B on the upper substrate of the panel, and displays desired images by controlling the transmittance of light therethrough. However, an LCD using the color filter method needs three times more pixels as compared with an LCD for displaying black and white because unit pixels corresponding to red R, green G, and blue B colors are needed. Therefore, a delicate panel manufacturing technology is needed to obtain high resolution images. Further, an LCD using the color filter method is difficult to manufacture because a color filter for each of red R, green G, and blue B colors needs to be formed.
An LCD using the field sequential driving method turns on light sources for red R, green G, and blue B lights sequentially and periodically, and displays color images by supplying the pixel voltage Vp to each pixel at each turning on period. The LCD using the field sequential driving method does not divide each pixel into unit pixels of red R, green G, and blue B colors, but displays three primary colors of red R, green G, and blue B using different color backlights by time-dividing the lights and sequentially applying them, and then expressing color images using eye's afterimage effect.
The LCD using the field sequential driving method drives one frame by dividing it into an R field, a G field, and a B field, and thus the response speed of the liquid crystal of LCD should be faster than that of the LCD using the color filter method. But since the response time of liquid crystal in a portable device (i.e., cellular phone), which is frequently exposed to low temperatures, decreases as the temperature decreases, a problem occurs in which the color reproducibility is degraded according to a decrease of response time when using the field sequential driving method.